1. Field of the Invention
This invention relates generally to friction stir joining methods. More specifically, the present invention is a method of joining metal work pieces together using a consumable friction stir tool that has at least a partially consumable pin, wherein the pin has a cutting edge that cuts through a first work piece material when rotated at a first speed. After at least cutting through the first work piece material to a sufficient depth, the rotational speed of the tool is changed to cause plasticization of the pin itself, as well as the first and second work piece materials being joined. After sufficient heating of the first and second work piece materials and the pin, the rotation of the tool is rapidly decelerated or stopped completely to enable the bonding of the pin and the first and second work piece materials. This process will be referred to throughout this document as friction stir riveting.
2. Description of Related Art
There are many methods for joining metal work pieces together; some of which include welding, spot welding, fasteners (such as screws and bolts), friction stir welding, etc. The three fundamental principles that govern all joining methods include mechanical attachment, fusion joining (welding), and solid state joining (friction welding). Each principle technique has advantages; however the method often selected for an application is dictated by the one having the fewest tolerable disadvantages.
Examples of mechanical work piece joining methods include screws, nuts and bolts, dove tail, swaging, riveting, interference attachment, etc. Many applications cannot use screws or bolts because the threads have limiting load carrying capability, the high cost of multiple components and assembly, the cost of the hole that must be placed in the work pieces and/or the space required for the fasteners. Dove tail and other work piece locking methods lock in specified directions but can slide or rotate apart in other directions. Rivets have perhaps the greatest joining strength per unit area and volume of any mechanical fastener but the mechanical deformation of the rivet head reduces the energy absorbing capability as well as elongation.
When mechanical methods are not acceptable joining techniques, fusion welding methods are utilized unless the work pieces are not considered weldable. For example, aircraft components made from 7000 series aluminum are not considered weldable because the resulting weld strength is as low as 50% of base metal properties. High melting temperature materials (HMTM) such as steel, stainless steel and nickel base alloys can be welded but the joint strength is limited to problems associated with fusion welding. These problems include, but are not limited to, solidification defects, hard/soft zones within the weld macrostructure, residual stresses resulting from liquid to solid phase transformation, porosity, cracking, non-uniform and unpredictable microstructures, corrosion susceptibility, work piece deformation, and loss of work piece base material properties. Post weld operations are often needed to repair distortion or evaluate the weld nondestructively and add cost to the process. In addition, there are health issues related to hexavalent chromium and manganese exposure as well as potential retina damage to the operator if proper safety procedures are not followed. In many cases, work pieces must be increased in size to use a base material of lower strength that is considered weldable in favor of a higher strength material that is not considered weldable. This is the case with automobile car bodies that are currently manufactured from lower strength steels. Advanced high strength steels (Dual Phase and TRIP steels) could be used in the frame construction to dramatically lower vehicle weight but these materials have not been used because of fusion weldability issues.
Friction stir welding (FSW) is a solid state welding process that has many advantages over fusion welding methods. FIG. 1 is a perspective view of a tool being used for friction stir welding that is characterized by a generally cylindrical tool 10 having a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a work piece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized work piece material. The work piece 16 is often two sheets or plates of material that are butted together at a joint line 18. The pin 14 is plunged into the work piece 16 at the joint line 18. Although this tool has been disclosed in the prior art, it will be explained that the tool can be used for a new purpose. It is also noted that the terms “work piece” and “base material” will be used interchangeably throughout this document.
The frictional heat caused by rotational motion of the pin 14 against the work piece material 16 causes the work piece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge. The result is a solid phase bond 20 at the joint line 18 that may be generally indistinguishable from the work piece material 16 itself, in comparison to other welds.
It is observed that when the shoulder 12 contacts the surface of the work pieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin 14. The shoulder 12 provides a forging force that contains the upward metal flow caused by the tool pin 14.
During FSW, the area to be welded and the tool are moved relative to each other such that the tool traverses a desired length of the weld joint. The rotating FSW tool provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the base metal, while transporting metal from the leading face of the pin to its trailing edge. As the weld zone cools, there is typically no solidification as no liquid is created as the tool passes. It is often the case, but not always, that the resulting weld is a defect-free, re-crystallized, fine grain microstructure formed in the area of the weld.
Travel speeds are typically 10 to 500 mm/min with rotation rates of 200 to 2000 rpm. Temperatures reached are usually close to, but below, solidus temperatures. Friction stir welding parameters are a function of a material's thermal properties, high temperature flow stress and penetration depth.
Previous patents by some of the inventors such as U.S. Pat. Nos. 6,648,206 and 6,779,704 have taught the benefits of being able to perform friction stir welding with materials that were previously considered to be functionally unweldable. Some of these materials are non-fusion weldable, or just difficult to weld at all. These materials include, for example, metal matrix composites, ferrous alloys such as steel and stainless steel, and non-ferrous materials. Another class of materials that were also able to take advantage of friction stir welding is the superalloys. Superalloys can be materials having a higher melting temperature bronze or aluminum, and may have other elements mixed in as well. Some examples of superalloys are nickel, iron-nickel, and cobalt-based alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium.
It is also noted that the phrase “friction stir processing” may also be referred to interchangeably with solid state processing. Solid state processing is defined herein as a temporary transformation into a plasticized state that typically does not include a liquid phase. However, it is noted that some embodiments allow one or more elements to pass through a liquid phase, and still obtain the benefits of the present invention.
In friction stir processing, a tool pin is rotated and plunged into the material to be processed. The tool is moved transversely across a processing area of the material. It is the act of causing the material to undergo plasticization in a solid state process that can result in the material being modified to have properties that are different from the original material.
The main disadvantage with FSW is a remaining hole left in the work pieces at the end of the weld. In many cases this is not a problem since a run off tab can be used at the end of the weld and later removed. A retractable pin can be used as the weld progresses to eliminate the end hole; however the tool and equipment requirements are extensive and costly. Tool geometries that allow the tool to be extracted from the weld gradually during FSW can also be used but the added process time combined with the added heat cycle over an existing weld increases cost and decreases base metal properties.
Friction stir spot welding (FSSW) is now being used experimentally to join advanced high strength steels in lap welding configurations. FSSW is being used commercially to lap weld aluminum components as described in US Patent application 20050178817. Two approaches are currently used.
The first approach involves plunging a pin tool (a FSSW tool comprised of a pin and a shoulder) into work pieces until the work pieces are spot friction welded together. The disadvantage with this method is the hole 26 left behind from the pin as shown in FIG. 2. The bond between the work pieces 28 is achieved under the shoulder of the tool while the pin hole reduces the strength of the weld.
A second method involves the design of equipment to force material back into the pin hole (U.S. Pat. No. 6,722,556). This method is quite cumbersome because of the large spindle head, fixturing requirements, and loads needed to make a spot weld.
It would be an advantage over the state of the art in the joining of metal work pieces to be able to provide and system and method that uses a partially consumable tool to perform FSSW using a rivet in a rapid and economical manner.